In previous years we studied homologous and hybrid fragment complexes of horse, tuna, candida and yeast iso-1 cytochromes c (cyts. c). This suggests that hydrophobic collapse of the cyt. c polypeptide chain containing the covalently bound heme that generates the bulk of the hydrophobic core surrounding the heme stabilizes the protein. Others have reported that horse cyt. c is more stable by 4.2 kcal/mol than yeast iso-2 cyt. c. Studies in previous years have also shown that the above hydrophobic collapse is associated with generation of stability difference between horse and yeast iso-1 or Candida cyt. c. The stability of the latter two cyts. c is also distinctly lower than horse cyt. c. Thus, to see whether the hydrophobic core residues alone can account for the stability difference between horse and yeast iso-2 cyts. c we prepared a chimera. In this we mutated all core residues of yeast iso-2 cyt. c that are different from horse cyt. c to those found in horse cyt. c. This chimera is referred to as horse core-yeast iso-2 shell chimera. This chimera has shown no significant increase in the stability over the wild type (wt) iso-2 cyt. c or an increase that is far from that expected if the stability is equivalent to horse cyt. c. Thus, the surface residues must be involved in the marked difference in the stability between horse and yeast iso-2 cyt. c. To see whether the core residues interact with the surface residues to generate stabilizing energy (core-shell interactions), we have constructed a complete thermodynamic cycle consisting of 4 elements. Element 1 is wt yeast iso-2 cyt. c. Element 2 is iso-2 cyt. c whose core residues are mutated to those of yeast iso-1 cyt. c variant containing Ala at position 102. Element 3 is iso-2 cyt. c whose surface residues are mutated to those of the iso-1 cyt. c variant. Element 4 is iso-2 cyt. c whose core and surface residues are mutated to those of iso-1 cyt. c variant. The 4 N- terminal residues that are missing in iso-1 cyt. are present in all elements. The samples of elements 1, 2 and 3 were prepared in previous years. Measurements of thermal transition of the 695 nm absorbance of these four element samples suggest that there may be core shell interactions that stabilize the element 4 over element 1.